SUBMITTED BY ARTHUR N1ORC - AMSAT A/c#31468
Answers to E-mail Questions About Foam Loss
NASA has received many e-mails at rtfsuggestions@nasa.gov with
suggestions or questions about foam loss from the Shuttle's External
Tank. Here are answers to some of the most common questions:
Is there a problem with the foam adhering to the aluminum surface of the
External Tank?
Our primary failure mode is cohesive failure of the foam; we have
experienced little, if any, adhesive failure of the foam not sticking,
per se, to the tank. Cohesive failure of the foam is different from the
adhesive failure mode in that the foam itself fails to stay together and
portions of the foam breaks free. Cohesive failure can be caused on a
small by build up in pressure of the closed-cell foam that we use as the
foam is heated during ascent. We refer to this phenomenon as
"pop-corning." Voids that form within the foam during application to the
tank can also lead to cohesive failure. Air entrapped within the voids
can expand with the heating experienced during ascent, increasing the
pressure, and ultimately cohesively failing the foam between the void
and the foam surface. Note that the delta pressure (change in pressure)
across the foam between the void and the surface is not only influenced
by ascent heating, but also by the ever-decreasing ambient pressure
until the vacuum of space is realized. Where voids form near or at the
interface between the foam and the tank structure, entrapped air will be
liquefied in the presence of the Liquid Hydrogen or Liquid Oxygen
temperatures at the tank's aluminum surface, the potential for cohesive
failure is exasperated.
Why doesn't NASA apply paint, a cover, or net over the tank?
One might remember that we painted the first couple of External Tanks
with white paint in the early 1980's. In both cases, we had a
significant amount of foam loss during ascent. Although at face value
applying a net or some other foam entrapping method to the External Tank
sounds easy, it is not without concern. After careful examination of
this approach, NASA's conclusion is that portions of the net could
become in itself an undesirable debris source. Depending on the material
used (Kevlar, aluminum, etc.), the density of the netting material would
present a more critical debris source than foam to the Orbiter Thermal
Protection System. Through a rigid certification process, we would also
have to understand if and when the netting material could come off and
in what quantities or mass that the netting material could present. Our
assessment is that the process of certifying a netting material for
flight would take several years and would not be available until late in
the Space Shuttle Program life. NASA's goal remains to eliminate the
potential for critical debris from all sources, including the External
Tank foam.
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